DTMF (dual tone multi frequency)

DTMF (dual tone multi frequency) is the signal to the phone company that you generate when you press an ordinary telephone's touch keys. In the United States and perhaps elsewhere, it's known as "Touchtone" phone (formerly a registered trademark of AT&T). DTMF has generally replaced loop disconnect ("pulse") dialling. With DTMF, each key you press on your phone generates two tones of specific frequencies. So that a voice can't imitate the tones, one tone is generated from a high-frequency group of tones and the other from a low frequency group. Here are the signals you send when you press your Touchtone phone keys:

Digit

Low frequency

High frequency

1

697

1209 Hz

2

697

1336

3

697

1477

4

770

1209

5

770

1336

6

770

1477

7

852

1209

8

852

1336

9

852

1477

0

941

1336

*

941

1209

#

941

1477

A number of companies make microchips that send and receive DTMF signals. The Telephony Application Program Interface (TAPI) provides a way for a program to detect DTMF digits.

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If you look at the table above you will see there is a pool of 4 low frequencies or tones and a pool of 4 higher frequency tones - giving a total of 16 unique combinations. The human voice can only re-produce one tone at a time, agreed that tone is infinitely variable but at any given point in time, it is only one frequency.

Chrissy24893's analogy of the piano is spot on. You can most likely tonally match (sing or whistle) each single key press of the piano (think do-ray-me-far-so-tee-la-do) but you couldn't reproduce do and tee together

This isn't a computer program. this is a combination of sounds (much like pressing more than one key on a piano at the same time) that occurs any time you press a number on any telephone's handset. it's a combination of tones so that a human voice cannot imitate it. DTMF replaced the Pulse Dialing of the old rotary phones.